She is television's "Queen of Shops" and known for her straight-talking, no-nonsense approach. She has turned some of Britain's grimmest, near-bankrupt small shops into stylish money-making businesses, taught basic retailing skills to charity shop workers and upset other store managers through her undercover exposés of shoddy customer service.

But she has never taken on a task of quite this magnitude. David Cameron will today ask Mary Portas to come up with a plan to help reverse the fortunes of the UK's high streets, handing her an ambitious brief that will aim to address soaring shop vacancy rates, prevent the proliferation of "clone towns" dominated by familiar big names and boost the number of small and independent retailers in town centres.

The 48-year-old management consultant and reality TV star has her work cut out. The UK retail sector generates 8% of the UK's GDP and employs 10% of the entire workforce. But high street outlets have been hit hard by the combination of the recession and the growth in online shopping. Large chains such as Woolworths, Zavvi and Borders have closed without new tenants moving into the space. Last week the electricals chain Comet said it was to close stores and shut a dozen regional service centres, and yesterday the accountants Ernst & Young warned of a decade of gloom for high street retailers as shoppers struggle with falling average incomes and try to pay down debt.

Central to the review – ordered by the Department for Business, Innovation and Skills – is the worrying level of sales in town centres, which have dramatically under-performed other locations. Government figures show that between 2005 and 2010, town-centre sales grew by a paltry 1.5%, while those in out of town shopping centres grew 11.5% and non-store (mail order and internet) soared by 71.5%.

In an interview last week with the Grocer magazine, Portas gave a hint of her initial thinking when she said that huge retailers such as Tesco should help their smaller counterparts: "What would be so wrong if out of all the millions in profit that Tesco makes every minute they were to put a percentage back into developing local community high streets? Consumers are starting to expect big businesses to support our towns as opposed to just driving in. Why would that be so difficult to do?"

She must deliver the rescue plan in the autumn. "Change needs to come," she said. "I'm working with the government because this is what I believe we need to do."

But critics may question whether Portas has the right credentials for the job, as she has never single-handedly run a major retailer. She joined Harrods as a window dresser in 1982, before moving to Topshop and then Harvey Nichols in 1990, where she helped transform it into an edgy fashion store and persuaded Jennifer Saunders to namecheck it endlessly in Absolutely Fabulous. In 1997 she left to set up Yellowdoor, a retail consultancy whose clients include the Oasis fashion chain, Westfield shopping centre and cosmetic brand Clinique.

Her appointment is also the latest in a slew of government advisory jobs handed to high-profile individuals with questionable results. In 2009 the then Labour government asked entrepreneur and The Apprentice boss Sir Alan (now Lord) Sugar to head a review of enterprise and Lastminute.com founder Martha Lane Fox was recruited as its digital champion. In August last year the founder of the Arcadia retail giant, Sir Philip Green, was asked by David Cameron to carry out a review of government spending and procurement.

Retailers welcomed the appointment but stressed they were keen to see an even-handed approach. Stephen Robertson, director general of the British Retail Consortium, said: "High streets are the heart of local communities and economies, providing jobs and essential services, but some are in trouble. The Government is right to recognise the future of our high streets cannot be left to chance but it must take a positive approach that supports retailers of all types and sizes."